Biostratigraphy
Biostratigraphy is the branch of stratigraphy that uses fossils to establish relative ages of sedimentary rocks and correlate them between and within basins. A biozone is an interval of strata characterised by certain fossil taxa. A biochron is an interval of time represented by certain taxa. Fossils provide useful information not only about the age of sediments but also the environment in which they lived and the prevailing climate at the time. Over short time intervals (in the order of a few tens of thousands of years or less), the presence or absence of species, or their relative abundance, is dictated more by environmental factors rather than the passage of time.
Upper left: Charophyte oogonium
Upper right: Diatom frustule
Lower left: Benthic foraminifer (thin section)
Lower right: Ostracod (left valve)
Micropalaeontology
Micropalaeontology is the study of a wide variety of biological groups usually between 0.1mm and 1mm in size, sometimes a little smaller and exceptionally larger.
In marine sediments the emphasis is usually on foraminifera (single-celled protists with a shell) a distinction being made between planktonic (floating in the upper water layers of oceans) and benthonic (bottom dwelling) ones.
Ostracods (arthropods with a carapace consisting of two valves) are mainly benthonic and are also frequently found in marine sediments in in some cases provide a detailed biozonation.
Radiolaria (planktonic marine protists with a siliceous skeleton), diatoms (siliceous skeletons of microscopic algae) and other groups are also studied. Non-marine sediments typically yield ostracods, diatoms and charophyte oogonia (fructifications of calcareous algae). Biozonation schemes which help to date sediments have been erected for all of these groups which are also useful in palaeoenvironmental interpretations.
Upper left: Eprolithus floralis
Upper right: Braarudosphaera africana
Lower left: Reinhardites levis
Lower right: Gartnerago segmentatum
(images courtesy of Network Stratigraphic)
Nannopalaeontology
Nannopalaeontology is the study of the calcite skeletons of single-celled marine planktonic algae, less than 30µm across and usually between 5 and 10µm which is one or two orders of magnitude smaller than that of microfossils. They are very easily extracted from sediments. They have a widespead distribution in marine sediments and a very large number of individuals can be found in a very small amount of sediment. Their calcareous skeletons are found in marine deposits often in vast numbers, sometimes making up the major component of a particular rock, such as the chalk of England.
However, because of their small size they can be easily reworked and there is also a risk of contaminiation. Their preservation is compromised in deep water sediments below the CCD.
Nannofossils are extremely useful biostratigraphically in sediments of Traissic (Carnian) and younger age, and detailed biozonation schemes have been set up.
Upper left: pollen grain
Upper right: dinoflagellate cyst
Lower left: dinoflagellate cyst
Lower right: dinoflagellate cyst
Palynology
Palynology is the study of organic-walled fossils including pollen, acritarchs, spores, dinoflagellate cysts, scolecodonts and chitinozoans, found in sediment and sedimentary rock. Specimens are usually between 5 and 100µm in size and in most cases contain sporopollenin, a compound very resistant to decay.
Pollen is produced from seed plants, of which the main types are “gymnosperms” (mainly conifers and related plants such as pine) and “angiosperms” (other flowering plants such as oak, grasses etc.). Spores are produced from ferns, mosses and some fungi.
Dinoflagellates form an important part of the microphytoplankton of marine and nonmarine waters and are useful in stratigraphic and environmental interpretations.
Palynology is a useful tool in that it can be applied in a variety of marine and non-marine settings. Most of these fossil groups can be used as indicators of age (using known first appearance and extinction events or relative abundances).
By recording the abundances of the various types in any sedimentary section, it is possible to establish the kind of vegetation present when the sediments were deposited, and in turn to make an interpretation about the climate.